1. Field of the Invention
This invention relates to refrigeration cycles and heat pumps and more particularly to systems of the vapor compression type.
2. Description of the Prior Art
The function of both refrigeration cycles and heat pumps is to remove heat from a source or reservoir at low temperature and to reject the heat to a sink or reservoir at high temperature. While many thermodynamic effects have been exploited in the development of heat pumps and refrigeration cycles, the most popular today is the vapor compression approach. This approach is sometimes called mechanical refrigeration because a mechanical compressor is used in the cycle.
Vapor compression refrigeration cycles generally contain five important components. The first is a mechanical compressor which is used to pressurize a gaseous working fluid. After proceeding through the compressor, the hot pressurized working fluid is condensed in a condenser. The latent heat of vaporization of the working fluid is given up to a high temperature reservoir often called the sink. The liquefied working fluid is then expanded at constant enthalpy in a thermal expansion valve or orifice. The cooled liquid working fluid is then passed through an evaporator. In the evaporator, the working fluid absorbs its latent heat of vaporization from a low temperature reservoir often called a source. The last element in the cycle is the working fluid itself.
In conventional vapor compression cycles, the working fluid selection is based on the properties of the fluid and the temperatures of the heat source and sink. The important factors in the selection include the specific heat of the working fluid, its latent heat of vaporization, its specific volume and its safety. The selection of the working fluid affects the coefficient of performance of the cycle.
For a refrigeration cycle operating between a lower limit, or source temperature and an upper limit or sink temperature, the maximum efficiency of the cycle is limited to the Carnot efficiency. The efficiency of a refrigeration cycle is generally defined by its coefficient of performance. The coefficient of performance is the quotient of the heat absorbed from the sink divided by the net work input required by the cycle.
For example, if the source and sink temperatures are 5.degree. F. and 86.degree. F. respectively, the Carnot coefficient of performance is 6.74. If the working fluid is refrigerant 11 (trichloromonofluoromethane) then the coefficient of performance is a maximum of about 5.04. If refrigerant 717 (ammonia) is selected, the maximum coefficient of performance is about 4.76. (See "Properties of Commonly used Refrigerants", Air Conditioning and Refrigeration Research Institute, 1957 ed.) The coefficient of performance associated with the working fluid varies with the required value of the source and the sink temperatures.
Aside from the thermodynamic limitations of the process, conventional mechanical refrigeration requires a relatively large, heavy compressor. The compressor generally has a great number of moving parts which are susceptible to wear. In general, mechanical compressors of the reciprocating, rotary or centrifugal type have volumetric efficiencies which are inversely related to the quantity of gas pumped. Hence a large refrigeration system employing a large compressor will have a higher efficiency than a small system pumping a small amount of working fluid.
There are many applications where the amount of heat to be removed is small and where a bulky mechanical compressor is undesirable. In these applications the designer might choose a vortex tube or a jet compressor. These devices are readily fabricated in small sizes and make use of the relatively high coefficients of performance attainable with vapor compression cycles. They also have the advantage of having no moving parts but they have the disadvantage of low compressor efficiency. For still smaller applications, Peltier effect devices have been employed. These devices, which do not employ a vapor compression principle, often have coefficients of performance less than 0.5.
In all refrigeration devices, the coefficient of performance of the device is inversely proportional to the heat removal capacity of the cycle. Another limitation of the prior art is associated with the control of compressor flow. When a refrigeration load varies, but the source and sink temperatures remain constant, it is desirable to reduce the flow of working fluid through the compressor while simultaneously maintaining the pressure rise across the compressor. This is a difficult job because in a mechanical compressor, the pressure rise and the flow rate are linked by the physical size of the parts of the compressor. In reciprocating compressors, this difficulty is surmounted by unloading cylinders. Unloading cylinders means the removal of flow from one or more of the cylinders in the compressor. This control mode results in very complex control systems and generally degrades the performance of the compressor.